1. Field of the Invention
This invention relates to an ink-jet ink composition for use in an ink jet printer, which performs a recording operation by use of a liquid ink composition. This invention particularly relates to a photo-polymerizable ink-jet ink composition, which is capable of being cured by irradiation of light.
2. Description of the Related Art
Ink jet recording techniques are printing techniques, wherein a liquid ink composition having a high fluidity is jetted out from fine nozzles and adhered to a recording medium, such as printing paper, and a print is thus formed. The ink jet recording techniques have advantages in that an image having a high resolution and good image quality is capable of being printed quickly with low noise and by use of a comparatively cheap apparatus. By virtue of the advantages described above, recently, the ink jet recording techniques have quickly become popular.
It is required for ink compositions utilized for the ink jet recording techniques to have various characteristics, such as a low viscosity, a surface tension sufficient for the formation of small liquid droplets jetted out from the nozzles of ink jet heads, a low volatility, and a good long-term stability.
Photo-polymerizable ink-jet ink compositions, which fall within one category of the ink-jet ink compositions, comprise a photo-polymerizable resin, a photo-polymerization initiator, and the like. Immediately after the printing operation, the photo-polymerizable ink-jet ink compositions are capable of being cured by the irradiation of light. Therefore, the photo-polymerizable ink-jet ink compositions have good drying characteristics and are free from the risk of an ink offset to the back of the printing paper. Since the photo-polymerizable ink-jet ink compositions are the ink compositions for use in the ink jet recording techniques, the characteristics, such as a sufficient surface tension, a low volatility, and a low viscosity, are required of the photo-polymerizable ink-jet ink compositions. Also, in particular, besides the characteristics described above, a high film strength and a low cure shrinkage are required of the photo-polymerizable ink-jet ink compositions, which are capable of being cured immediately by the irradiation of light.
It is currently considered that, in order for the ink compositions to be jetted out by use of ink jet heads for photo-polymerizable ink compositions, which ink jet heads have been developed for use in industrial applications, the viscosities of the ink compositions should be as low as at most 50 cps. Examples of the photo-polymerizable ink-jet ink compositions having the low viscosities described above include an ink composition using a solvent, as described in Japanese Unexamined Patent Publication No. 62(1987)-64874, and an ink composition using a low-viscosity resin, as described in U.S. Pat. No. 6,593,390.
However, with the ink composition using a solvent, as described in Japanese Unexamined Patent Publication No. 62(1987)-64874, since a volatile solvent, such as methyl ethyl ketone or ethanol, is utilized, the problems occur in that, accompanying the volatilization of the solvent, the viscosity of the ink composition becomes high, agglomeration of a coloring material occurs, and clogging of the ink jet head occurs. Therefore, the problems occur in that the long-term stability cannot be kept good. Also, the ink composition using the volatile solvent is not appropriate from the view point of keeping environmental loads low.
Also, the ink composition using a low-viscosity resin, as described in PCT Japanese Publication No. 2001-525479, is an ink composition containing a monomer and an oligomer. However, with the ink composition using a low-viscosity resin described above, such that the viscosity of the ink composition may be kept low, the proportion of the monomer having a small number of functional groups, such as a mono-functional monomer or a bi-functional monomer, is set to be high, and the proportion of the oligomer is set to be low. Therefore, though the viscosity of the ink composition is capable of being kept low, the problems occur in that the functionality of the resin as a whole cannot be kept high due to the low proportion of the oligomer, and in that the film strength cannot be kept high due to a low crosslinking density. As described above, in the cases of the ink composition using a low-viscosity resin, the constitution required of the ink composition for keeping the viscosity low and the constitution required of the ink composition for keeping the film strength high are incompatible with each other. Therefore, the photo-polymerizable ink-jet ink composition is produced at a certain level of mutual sacrifice of the viscosity and the film strength.